After sitting for a while the game will not boot at power-on (lights come on, no displays). Switching it off and on a few times gets it into audit mode and then a normal boot, after which it plays fine. A new battery pack, 5101 RAM and socket, and the memory-protect diode did not help.

Game only boots after several power cycles: corrosion under the 6808 CPU socket

Cause: A small spot of corrosion under the 6808 processor socket had darkened a few socket pins, giving a marginal contact that failed on a cold start.

Checks

• Check 5 V and 12 V at the CPU board, and measure the 5 V on the AC range: under about 0.5 V AC means the filter capacitor is fine.
• With the game off, confirm battery voltage reaches pin 22 of the 5101 RAM.
• Pull the 6808 and inspect its socket pins and the board under the socket for corrosion.

Fix

• Replace the 6808 socket and clean any corrosion from the board under it.

Systems

• Williams System 6 / 6A

Games

• Alien Poker

Related tips

• Game powers up in audit mode instead of attract mode
• Game keeps resetting: replace the 5101 RAM (an NVRAM works)
• Intermittent lockups with both CPU LEDs on
• "Adjust Failure" at power-on
• What changed on the System 6A CPU board
• CPU never starts: check the reset line at IC1 pin 40
• Random lockups from the 12 V logic pin
• Usual causes of lock-ups and resets

Source: Pinball Repair help group!! (Facebook group)